In the process of manufacturing a semiconductor device, liquid perfluorocarbons (hereinafter referred to as “PFCs”) having an excellent electric insulating property have been heretofore widely used as the heat medium for controlling the wafer temperature within a chamber in an etching process using a high-voltage plasma or for maintaining a high temperature or a low temperature in the thermal shock test of a wafer or in the performance inspection of the wafer.
Recently, a problem of global warming has been raised and based on the protocol adopted at the Kyoto conference in 1997, Japan has also started to establish goals for reducing the discharge of six kinds of gases including PFCs and to take a specific action.
PFCs are very stable (inactive), both thermally and chemically, and are not considered to deplete the ozone layer because the ozone depleting potential (ODP) thereof is zero due to the absence of chlorine atom in the molecule. Moreover, PFCs are low toxic and noncombustible. By virtue of these excellent properties, they have been widely used, particularly, in the field of manufacture of semiconductor devices where corrosion resistance, insulating resistance and the like are required. Representative examples of liquid PFCs include C5F12, C6F14 and C8F18. These PFCs are, however, known to have a great effect on warming because of their very high stability and the global warming potential (GWP) thereof are as high as 5,000 to 7,000 (integral term: 100 years, assuming that CO2 is 1).
In the case of using these PFCs as the above-described heat medium in an open system or even in a closed system, since complete enclosure is difficult in view of the apparatus design or structure, the system is substantially opened to atmospheric conditions in many cases. Therefore, particularly when the temperature is close to the boiling point of the substance used, the substance inevitably volatilizes into the atmosphere in a large amount. This not only increases the cost necessary for adding and replenishing the heat medium but also raises a serious problem in view of the discharge of a warming gas.
Hydrofluoroethers (hereinafter referred to as “HFEs”) and hydrofluorocarbons (hereinafter referred to as “HFCs), which are considered to have a small effect on warming, have been developed as alternatives of PFCs. A representative example of HFEs is C4F9OC2H5 and a representative example of HFCs is cyclic C5H2F8. However, these substances are relatively low in boiling point (max.: about 80° C.), and therefore, they cannot be used at high temperatures which reaches 100° C., and the electric properties thereof are inferior to conventional PFCs. Because of these reasons, their use in the field of manufacturing of semiconductor devices is limited and these substances are mainly used as a cleaning agent or a solvent.
In the process of manufacturing a semiconductor device, the use form of the heat medium is classified into indirect heat transfer and direct heat transfer. Indirect heat transfer is where the heat medium itself circulates through a heat transfer path (e.g., heat exchanger) and repeats heat absorption or heat release. Direct heat transfer is where the temperature of a material body is changed or maintained while allowing the heat medium to stand in the state of directly contacting with the material body, for example, by dipping the material body directly in the heat medium. In either case, a wide temperature region from an extremely low temperature of about −50° C. to a high temperature of 100° C. must be continuously or intermittently covered. Therefore, there is a demand for a system of operating the transfer of heat by one kind of heat medium. For example, in the case of a substance which does not boil in a high temperature region (that is, a substance having a high boiling point of at least 100° C. or more), the substance is required to have properties of maintaining the liquid phase even in an extremely low temperature region of −50° C., which is included in the temperature range during use, and required to have appropriate flowing properties at the same time. For this purpose, the freezing point or the temperature where the substance starts flowing (pour point) is preferably lower than the lowest temperature on use.
In recent years, studies have been made on extremely low temperature conditions on the level of −70° C. and therefore, the substance must have the required properties at low temperatures. On the other hand, from the standpoint of coping with the warming problem, the substance must be reduced in volatilization loss by having the required properties at high temperatures. Accordingly, in practice, the range from the freezing point to the boiling point of the heat medium is preferably the temperature range ±30° C. during use, namely, approximately from −100 to 130° C. The existing HFEs and HFCs are known to have a relatively low freezing point but are inferior to PFCs in the electrical properties such as dielectric breakdown voltage and volume resistivity. On the other hand, PFCs are excellent in these electrical properties but those having a low freezing point are liable to have a small molecular weight and a low boiling point. As such, when conventional HFEs, HFCs and PFCs are used as a sole compound, they cannot be a preferred heat medium that is well-balanced in physical properties by practically having a broad temperature range in terms of the temperature range from the freezing point to the boiling point and having good electrical properties at the same time.
Therefore, for example, a method of mixing specific components to provide an azeotropic composition or an azeotrope-like composition to obtain a constant boiling mixture, or a method of adjusting the mixing ratio according to the use conditions to control the properties of the mixture may be developed. However, for using the heat medium in such a system, the constant boiling composition is indispensable, but the combination of existing HFEs, HFCs and PFCs cannot form an azeotropic composition. Moreover, the method of adjusting the mixing ratio highly probably incurs changes in the composition and this is disadvantageous in practice.
The present invention has been made under these circumstances. An object of the present invention is to provide a heat medium, which can contribute to the reduction in the discharge of warming gas by having high boiling properties, can maintain the liquid phase in a wide temperature range during use by having a low freezing point and, at the same time, is favored with excellent electrical insulating properties.
As a result of extensive investigations to solve the above-described problems, the present inventors have found that a constant boiling composition comprising perfluoro-1-propyl-3,4-dimethylpyrrolidine, perfluoro-1-propyl-3-methylpiperidine and perfluorotripropylamine exhibits excellent properties as a heat medium in the process of manufacturing a semiconductor device. Furthermore, extensive investigations have been made on the process for producing the composition. As a result, the present inventors have found that the constant boiling composition can be easily synthesized by using a triallylamine having high general use property and electrolytically fluorinating it in an anhydrous liquid hydrogen fluoride. The present invention has been accomplished based on these findings.